1. Field of the Invention
The present invention relates to an optical filter which is suitably employable as a gain equalizer for equalizing the gain of optical amplification of signal light, or the like in an optical amplifier.
2. Related Background Art
An optical amplifier includes an optical waveguide, doped with a fluorescent material excitable with pumping light, for optically amplifying signal light; and pumping means for supplying pumping light to the optical waveguide. Such an optical amplifier is provided in a repeating station in an optical transmission system, or the like. In particular, it is important for the optical amplifier employed in a wavelength division multiplexing transmission system for transmitting signal light of a plurality of channels to optically amplify the plurality of channels in a collective manner at respective gains equal to each other, and to output each channel of signal light with a power held at a constant target value. Hence, for equalizing the optical amplification of signal light in such an optical amplifier, an optical filter having a loss spectrum in a form similar to that of the gain spectrum in the amplifying optical waveguide is used as a gain equalizer.
For example, literature 1xe2x80x94K. Inoue, et al., xe2x80x9cTunable Gain Equalization Using a Mach-Zehnder Optical Filter in Multistage Fiber Amplifiers,xe2x80x9d IEEE Photonics Technology Letters, Vol. 3, No. 8, pp. 718-720 (1991)xe2x80x94and literature 2xe2x80x94H. Toba, et al., xe2x80x9cDemonstration of Optical FDM Based Self-Healing Ring Network Employing Arrayed-Waveguide-Grating ADM Filters and EDFAsxe2x80x9d, ECOC ""94, pp. 263-266 (1994)xe2x80x94disclose techniques for flattening the gain of an optical amplifier by means of an optical filter using a Mach-Zehnder interferometer. The techniques disclosed in these literatures aim at adjusting the respective temperatures of individual optical couplers and individual branching optical paths in a Mach-Zehnder interferometer according to the input signal light power, so as to regulate the slope of transmission loss to wavelengths in optical filter, thereby compensating for the fluctuation in the slope of gain to wavelengths accompanying the fluctuation in input signal light power.
In the above-mentioned conventional techniques, if the power of signal light outputted from the optical amplifier is to be kept constant when the loss in an optical transmission line in front of the optical amplifier fluctuates due to some reason and thereby the signal light fed into the optical amplifier alters its power, it will be necessary for the optical amplification of signal light in the optical amplifier to change its gain. When the gain is changed, the wavelength dependence of gain, i.e., the slope of gain to wavelengths (gain slope), may fluctuate, thereby deteriorating the gain flatness of optical amplifier, which causes the respective powers of a plurality of channels of signal light outputted from the optical amplifier to deviate from each other. Therefore, the respective temperatures of individual optical couplers and branching optical paths in each Mach-Zehnder interferometer constituting the optical filter are adjusted according to the input signal light power, so as to adjust the slope of loss to wavelengths (loss slope) in optical filter, thereby compensating for the fluctuation in gain slope accompanying the fluctuation in gain slope. When the loss slope in optical filter is changed according to the input signal light power, however, the loss level in the signal light wavelength band may fluctuate, whereby the signal light outputted from the optical amplifier after being optically amplified may fluctuate and deteriorate its S/N ratio.
In order to overcome the problems mentioned above, it is an object of the present invention to provide an optical filter which is suitably employable as a gain equalizer or the like in an optical amplifier, whereas its loss slope is easy to control.
For achieving the above-mentioned object, the optical filter in accordance with the present invention is an optical filter having variable wavelength-dependent transmission loss within given wavelength band comprising (1) an optical waveguide section composed of first, second and third Mach-Zehnder interferometers, each having two optical paths and two optical couplers for optical connecting said two optical paths each other at both end, these Mach-Zehnder interferometers are connected in series, and only one of the Mach-Zehnder interferometers having same optical path lengths between the optical couplers, (2) first, second and third temperature adjusting means, each disposed in at least one of optical paths between the optical couplers in respective first, second and third Mach-Zehnder interferometers, for adjusting the respective temperatures of the optical paths, and (3) control means, for independently controlling the absolute value and the slope of transmission loss by controlling respective temperatures of the optical paths with the temperature adjusting means.
This optical filter has transmission loss dependent on wavelength for light passing between the input terminal and the output terminal, and has a configuration of three Mach-Zehnder interferometers connected in series. Also, by adjusting the phase shift in each interferometer with adjusting the temperatures of each optical path using each respective temperature adjusting means, the absolute value of transmission loss in a given wavelength band (for example, the wavelength band of 1530 nm to 1565 nm, or alternatively, the wavelength band of 1574 nm to 1609 nm) and the slope of transmission loss for the wavelength are allowed to be controlled independently each other.
Alternatively, an optical filter according to the present invention is an optical filter having variable wavelength-dependent transmission loss within given wavelength band comprising (1) a single Mach-Zehnder interferometer composed of two optical paths having same optical path lengths and two optical couplers connected these optical paths each other, (2) a duplex Mach-Zehnder interferometer composed of main optical path connected one input or output end of the single Mach-Zehnder interferometer, sub optical path, end optical couplers disposed at both ends of the main and sub optical paths for optical coupling the main and sub optical path each other, and middle optical coupler disposed at middle position of the main and sub optical paths for optical coupling the main and sub optical path each other and the main optical path has different optical path lengths between adjacent optical couplers from the sub optical path each other, (3) a first temperature adjusting means disposed in at least one of optical paths between the optical couplers in said single Mach-Zehnder interferometer, for adjusting the temperature of the optical path, (4) second and third temperature adjusting means, each disposed in at least one of main or sub optical paths between the middle optical coupler and end optical couplers respectively in the duplex Mach-Zehnder interferometers, for adjusting the respective temperatures of the optical paths, and (5) controlling means, for independently controlling the absolute value and the slope of transmission loss by controlling respective temperatures of the optical paths with these temperature adjusting means.
This optical filter also has transmission loss dependent on wavelength for light passing between the input and the output terminal, and has a configuration of three Mach-Zehnder interferometer connected in series. Also, in this optical filter as well, by adjusting the phase shift in each interferometer with adjusting the temperature of each optical path using each respective temperature adjusting means, the absolute value of transmission loss in a given wavelength band (for example, the wavelength band of 1530 nm to 1565 nm, or alternatively, the wavelength band of 1574 nm to 1609 nm) and the slope of transmission loss for the wavelength are allowed to be controlled independently each other.
It is preferable that in an optical filter according to the present invention the bandwidth of the given wavelength band is 35 nm or wider, and the variable range of the absolute value of transmission loss in the given wavelength is 10 dB or more, and the absolute value of the slope of transmission loss in the given wavelength is variable at least within the range of 0 to 5 dB/35 nm.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.